Process for reducing surface checking during hot working of steel



United States Patent 3,382,700 PROCESS FOR REDUCING SURFACE CHECKlN G DURING HGT WORKING OF STEEL William E. Heitmann, Bolton, 111., and Michael George Wright, Hammond, Ind., assignors to Inland Steel Company, Chicago, 111., a corporation of Delaware No Drawing. Filed Mar. 31, 1966, Ser. No. 538,941 14 Claims. (C1. 72-364) The present invention relates generally to the hot working of unfinished steel articles such as blooms, billets, slabs, ingots and the like. More particularly, the invention relates to a process for minimizing or eliminating surface checking during the hot working of unfinished articles composed of steel containing a metallic alloying element, other than iron, and the inclusion of which, experience has shown, increases the likelihood of surface checking in the unfinished steel article during hot working.

Surface checking is manifest by a large number of small cracks at the surface of the steel article, particularly along corners. Severe surface checking renders an article commercially unacceptable.

Typical alloying elements, the inclusion of which in-' creases the likelihood of surface checking in an unfinished steel article during hot working, are tellurium and copper. A substantial proportion, if not all, of these two alloying elements are in solid solution with iron at typical hotworking temperatures, e.g., 2000-2400 F, a temperature range in which austenite grains constitute the matrix of the microstructure of the steel to which the present invention relates.

Preceding a hot-working step, an unfinished steel article is generally subjected to a heating step, to elevate the temperature of the steel to the desired hot-working temperature; and the atmosphere is oxidizing in a typical commercial furnace for heating the unfinished steel article.

Iron is much more readily oxidizable than copper or tellurium, so that, in that portion of the unfinished steel article adjacent the surface thereof, some of the iron is oxidized to form a scale of iron oxide on the surface of the article. Because some of the iron has been oxidized, the relative proportion of iron to the other metallic alloying element (e.g., tellurium or copper), in the solid solution, decreases in the surface-adjacent portion of the unfinished steel article; and the concentration of the other metallic alloying element increases. As oxidization of the iron continues, the concentration of the other metallic alloying element eventually increases above that which will remain in solid solution with iron; and some of the other alloying element comes out of solution. Such an increased concentration of said other metallic alloying element, during the heating step, increases the susceptibility of the unfinished steel article to surface checking during a subsequent hot-working step.

The problem of surface checking. as described above, is substantially minimized, if not entirely eliminated, in accordance with the present invention, by subjecting the unfinished steel article to an operation, after the heating step and before the hot-working step, during which the entire surface-adjacent portion of the article is removed to at least substantially the depth of the increased concentration of the other metallic alloying element.

Other features and advantages are inherent in the process claimed and disclosed or will become apparent to those skilled in the art from the following detailed description.

Copper is generally added to plain carbon steel, to improve corrosion resistance, in amounts which give the steel a copper content between 0.05 and 2.0 wt. percent.

A typical copper-containing steel, the hot-working properties of which may be improved by a process in accord- "ice ance with the present invention, has the following typical composition, in wt. percent:

Carbon 0.15 Manganese 0.75 Sulphur 0.025 Phosphorus 0.015 Silicon 0.25 Copper 1.0

Iron, essentially the balance.

An unfinished steel article, having the composition set forth above, is typically subjected to a heating step in conventional commercial apparatus usually provided for such a purpose. For example, an ingot would be subjected to heating in a conventional soaking pit while blooms or billets would be subjected to heating in conventional reheating furnaces.

The unfinished steel article is heated until the temperature of the steel article is, typically, between 2000" F. and 2400" F. In this temperature range, the micr0structure of the above-described steel has a matrix composed of austenite; and copper is dissolved in the austenite in this temperature range. However, because the atmosphere in the heating zone is oxidizing, and because iron is more readily oxidizable than copper, iron in the surface-adjacent portion of the article is oxidized and accumulates at the surface as iron oxide scale. As the iron oxidizes, the proportion of iron to copper, in the surface-adjacent portion of the article, decreases; and theconcentration of copper increases.

As the concentration of copper increases, there is a tendency for the copper to migrate toward the center of the unfinished steel article; but the rate of iron oxidation in the surface-adjacent portion of the article exceeds the rate of copper migration toward the center of the article, so that there is a relative increase in the concentration of copper in the surface-adjacent portion, especially near the interface between the iron oxide scale on the surface and the metallic steel of the surface-adjacent portion of the article.

Eventually, the concentration of copper reaches a level above that which will be maintained in solid solution, and at least some of the copper comes out of solid solution. Because the temperature to which the article has been heated is above the melting point of copper (198l -F.), the copper coming out of solid solution melts and begins to penetrate into the grain boundaries located between grains of austenite in the surface-adjacent portion of the steel.

The penetration of molten copper into the 'a-ustenite grain boundaries reduces cohesion between austenite grains and causes intergranular Weakness. If steel, with molten copper between grains of austenite, is hot worked, the reduced cohesion between 'aust-enite gnains initially results in the formation of fissures or cracks; and, with further deformation, these fissures increase in size until the condition known as surface checking takes place.

Removal of the entire surface-adjacent portion of the article, before hot working, to a depth to which the molten copper has penetrated, substantially minimizes the likelihood of surface checking during the subsequent ho tworking step.

Before deformation, the intergranular penetration of the molten copper extends below the interface, between the iron oxide surface scale and the metallic steel of the surface-adjacent portion, no more than several grain diameters. Therefore, by performing the surface-removal step before the hot-working step, it is possible to remove the zone of penetration of molten copper merely by removing metal to a relatively shallow depth. In a typical operation, metal would be removed to a depth of about 0.010 inch.

On the other hand, if the surface-adjacent portion of the unfinished article, having intergranular molten copper penetration, is not removed before the hot-working step, the penetration of molten copper between the austenite grains increases in depth during the hot-working step. Accordingly, performing a surface-removal step after hot working (e.g., following the deformation of a bloom to a billet which is to be subsequently reheated and then subsequently deformed, as a billet, into a finished article such as a bar) would require the removal of a substantially greater amount of material.

In the embodiment under consideration, the step of removing the surface-adjacent portion is per-formed while the steel article is at a hot-working temperature and before the performance of any deforming step; and this surface-removal .step is performed as quickly as possible because it is desired that the temperature of the article, at the time a subsequent hot-working step is initiated, be as close as possible to the tempenaure to which the article was heated during the heating step.

In an operation containing a number of hot-working steps, for example an operation in which an ingot is subjected to a hot-working step to form a bloom and the bloom is then subjected to a hot-working step to form a billet and the billet is then subjected to a hot-working step to form a bar, it is advantageous to subject each of the unfinished steel articles, namely the ingot, the bloom and the billet, to the same sequence of operations described above with respect to an unfinished steel article generally, to minimize surface checking. Thus, the ingot would be subjected to the three steps of heating, surface removal as described above, and hot working; the bloom would be subjected to the same three steps; and the billet would be subjected to the same three steps.

:In accordance with the present invention, surface removal may be accomplished by grinding or by other satisfactory surface-removal procedures heretofore used for the removal of surface-adjacent portions of unfinished steel articles. The important consideration is the removal of substantially the entire surface-adjacent portion of the unfinished article to the depth of increased concentration of the metallic alloying element other than iron. Thus, in grinding a billet having a square cross section, for example, the entire surface area of each side of the hot billet would be contacted by an abrading surface of the grinding tool and grinding would be performed until the required depth of material was removed from each side of the billet.

Tellurium is added to steel to increase the machinability of the steel; and the tellurium content may range between 0.02 and 0.50 wt. percent. Preferably, the tellurium content is between 0.02 and 0.07 wt. percent. Oftentimes, tellurium is added together with about 0.15-0.35 wt. percent lead, and/or between 0.25 and 0.35 wt. percent sulphur, to increase machinability.

Representative compositions of tellurium-containing steels are set forth in .Holow-aty, U.S. Letters Patent Nos. 3,152,889 and 3,152,890.

A typical tellurium-containing steel, the hot-working properties of which are improved in accordance with the process of the present invention, has the following composition, in wt. percent:

Carbon 0.08 Manganese 1.0 Sulphur 0.30 Phosphorus 0.05 Silicon 0.02 Tellurium 0.065 Iron, essentially the balance.

The sequence of operations for hot working a telluriumcontaining steel includes a conventional heating step during which the tellurium-containing steel is heated to a temperature typically in the range between 2000 F. and 2400 F. This is above the melting point of tellurium (840 F.), and approaches the boiling point of tellurium (2530 F.). Typically, the atmosphere is oxidizing in the zone in which the heating step is conducted; and because iron is more readily oxidizable than tellurium, the iron in the surface-adjacent portionof the article is oxidized, while the tellurium is not. At least a substantial portion of the tellurium is in solid solution with iron in austenite at the hot-working temperature to which the article is heated. As the iron is oxidized, the concentration of tellurium in the surface adjacent portion of the article increases. Eventually, the increased concentration of tellururn exceeds that which will remain in solid solution; and some of the tellurium comes out of solid solution. The increased concentration of tellurium in the surface-adjacent portion of the steel increases the likelihood of surface checking on the article during the hot-working step [following the heating step.

Surface checking can be minimized, if not entirely eliminated, by subjecting the tellurium-containing steel to a surface-removal step, following heating and before hot working, during which the entire surface-adjacent portion of the article is removed to substantially the depth of increased concentration of tellurium.

In a typical operation, tellurium-containing steel, having a composition similar to that set forth above, is heated to a temperature of 2200 F. After the heating step and before the hot-working step, the surface-adjacent portion of the article is removed, in its entirety, to a depth of at least 0.015 inch below the iron oxide scale which has formed on the surface. This assures an optimum surface condition on the hot-worked article, from the standpoint of absence of surface checking. If the surface-adjacent portion is removed to a depth less than 0.015 inch, the surface condition of the article becomes progressively worse as the depth of removal is progressively lessened.

The embodiments of the invention described above are useful in connection with improving the surface properties of hot-worked, unfinished steel articles containing metallic alloying elements, other than iron, which are less readily oxidizable than iron.

Another embodiment of the process of the present invention is applicable to unfinished articles composed of steel containing metallic alloying elements, other than iron, which are more readily oxidizable than iron and which, together with iron, form an oxide slag having a melting point below the temperature to which the unfinished article is heated during the heating step. In such a situation, the slag acts much in the same manner as does molten copper, in that the slag penetrates into the grain boundaries of the steel, setting up intergranular weaknesses which cause formation of fissures during hot working and produce surface checking on the article. This problem can be alleviated by subjecting the article to a surface-removal step, following heating and before hot working, during which the surface-adjacent portion of the steel, in its entirety, is removed to the depth of penetration of the molten slag.

A typical example of a steel, the hot-working properties of which may be improved by utilizing the last-described embodiment of the process of the present invention, has a composition as follows, in wt. percent:

Carbon 0.06 Manganese 0.30 Sulphur 0.025 Aluminum 0.40 Silicon 1.25

Iron, essentially the balance.

The steel described above is a low silicon steel used for preparing electrical sheets.

When a steel having the above composition is sub jected to a heating step in which the steel is heated to a temperature between 2000 F. and 2400 F., in a zone having an oxidizing atmosphere, not only the iron but also the silicon and aluminum are oxidized, resulting in the formation of an iron oxide-aluminum oxide-silicon oxide slag on the surface of the article. This slag has a melting point considerably below the temperature to which the article is heated, and this molten slag penetrates into the grain boundaries of the steel.

If the steel were subjected to hot working without a surface-removal step before the hot-working step, substantial surface checking would occur during the hotworking step. However, subjecting the article to a surface-removal step, following heating and before hot working, and in which the entire surface-adjacent portion of the article is removed to the depth of penetration of the molten slag, alleviates the problem of surface checking during the subsequent hot-working step.

A temperature of 2150 F. in an oxidizing atmosphere produces a molten slag consisting of iron oxide and silicon oxide. Inclusion of aluminum in the above composi tion, causing the formation of a much lower melting, aluminum oxide-containing slag, leads to surface checking problems in the absence of a process in accordance with the present invention.

Thus, in one embodiment, a process in accordance with the present invention is applicable to improve the hotworking properties of an unfinished article composed of steel containing a metallic alloying element, other than iron, which is less readily oxidizable than iron and which has a melting point below the temperature to which the steel article is heated during a heating step preceding the hot-working step. This embodiment is especially applicable where at least a substantial part of the metallic alloying element is in solid solution with iron, before the heating step.

In another embodiment, a process in accordance with the present invention is applicable to improve the hotworking properties of an unfinished article composed of steel containing metallic alloying elements, other than iron, and which are more readily oxidizable than iron and the oxides of which, together with the oxide of iron, form a slag which is molten at the temperature to which the steel article is heated in the heating step preceding hot working.

In all embodiments, the unfinished steel article is subjected to a surface-removal step, following the heating step and before the hot-working step, during which the entire surface-adjacent portion of the article is removed to the depth of increased concentration of the metallic alloying element and/or to the depth of penetration of the liquid phase which has penetrated into the grain boundaries of the steel article, said increased concentration and/or penetration occurring as a result of conditions prevailing during the heating step.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. In a process for hot working an unfinished article composed of steel containing at least one metallic alloying element in addition to iron, said process including the steps of:

heating said unfinished steel article to a hot-working temperature under conditions which, in that portion of the unfinished steel article adjacent the surface thereof, (1) oxidize at least some of the iron therein and (2) cause an increased concentration of at least one metallic alloying element of the steel other than iron;

and then deforming said unfinished steel article with the deformation being initiated while said article is at a hot-working temperature;

the improvement comprising the step of removing the entire surface-adjacent portion of said article to at least substantially the depth of said increased concentration;

said surface-removal step being performed between said heating and deforming steps;

whereby surface checking of said article, during said deforming step, is minimized.

2. In a process as recited in claim 1 wherein:

said surface-removal step is performed while the unfinished steel article is at a hot-working temperature, and before the article is subjected to any deforming steps following said heating step.

3. In a process as recited in claim 2 wherein said additional metallic alloying element is less readily oxidizable than iron and has a melting point lower than said hotworking temperature to which the article is heated during said heating step.

4. In a process as recited in claim 3 wherein:

at least a substantial portion of said additional metallic alloying element is in solid solution with iron in the surface-adjacent portion of the article, before said heating step;

and said heating step includes heating said article to a temperature above the melting point of the additional metallic alloying element under conditions which oxidize sufiicient iron in the surface-adjacent portion of the article to increase the concentration of the additional metallic alloying element in the surface-adjacent portion and cause at least some of the additional metallic alloying element to come out of solid solution.

5. In a process as recited in claim 4 wherein:

said additional metallic alloying element is copper, and

said copper is in solid solution with iron in the surface-adjacent portion of the article, before said heating step;

and said heating step includes heating said article to a temperature above the melting point of copper under conditions which oxidize sufiicient iron in the surface-adjacent portion of the article to cause at least some of the copper to come out of solid solution and melt and pentrate into the grain boundaries of said surface-adjacent portion;

said surface-adjacent portion being removed to substantially at least the depth of penetration of the molten copper.

6. In a process as recited in claim 5 wherein:

said unfinished steel article contains between 0.05 and 2.0 Wt. percent copper;

and said surface-adjacent portion is removed to a depth of several grain diameters.

7. In a process as recited in claim 4 wherein:

said additional metallic alloying element is tellurium.

8. In a process as recited in claim 7 wherein said steel contains 0.02-0.07 wt. percent tellurium.

91. In a process as recited in claim 7 wherein:

said steel is heated to about 2200" R;

and said surface-adjacent portion is removed to a depth of at least 0.015 inch.

10. In a process for hot-working an unfinished article composed of steel containing metallic alloying elements in addition to iron, said process including the steps of:

heating said unfinished steel article to a hot-working temperature under conditions which, in that portion of the unfinished steel article adjacent the surface thereof, (1) oxidize at least some of the iron therein, (2) permit formation of a molten phase containing at least one metallic alloying element of the steel other than iron, and (3) permit said molten phase to penetrate into the grain boundaries of said surfaceadjacent portion of the unfinished steel article;

and then deforming said unfinished steel article with the deformation being initiated while said article is at the hot-working temperature;

the improvement comprising the step of removing the entire surface-adjacent portion of said article to at least substantially the depth of penetration of said liquid phase;

3,382,700 7 8 said surface-removal step being performed between said some of the iron has been oxidized and at least said one heating and deforming steps and while the unfinished metallic alloying element other than iron has been insteel article is at a hot-working temperature. creased in concentration, the steps of:

11. In a process as recited in claim 10 wherein:

first removing the entire surface-adjacent portion of one of said additional metallic alloying elements is 5 said article to at least substantially the depth of said copper; 7 increased concentration; and said molten phase is composed of copper. and then deforming said unfinished steel article at a 12. In a process as recited in claim 10 wherein: hot-Working temperature. said additional metallic alloying elements are silicon 14. In a process as recited in claim 1 wherein:

and aluminum; 10 said one metallic alloying element is tellurium; said heating step includes heating said article to a temsaid unfinished steel article is a billet;

perature above the melting point of a slag containand said removing step is performed before deforming ing iron oxide and silicon oxide plus aluminum oxide; said billet into a bar. and said heating step is performed under conditions which (1) oxidize aluminum, silicon and iron in the 15 References Cited surface-adjacent portion of the article and (2) cause UNITED STATES PATENTS the formation of a molten slag, containing iron oxide and silicon oxide plus aluminum oxide, which 2 Z 9/1941 Rohn et 3 2 penetrates between the grain boundaries of said sur- Z 5/1957 Than face-adjacent portion. 20 2,968,09- 1/1961 Ohtake et al 72-700 13. In a process for hot working an unfinished article RICHARD J. HERBST, Primary Examiner.

L. A. LARSON, Assistant Examiner.

composed of steel, wherein said steel contains at least one metallic alloying element in addition to iron and said unfinished article has a surface-adjacent portion in which 

1. IN A PROCESS FOR HOT WORKING AN UNFINISHED ARTICLE COMPOSED OF STEEL CONTAINING AT LEAST ONE METALLIC ALLOYING ELEMENT IN ADDITION TO IRON, SAID PROCESS INCLUDING THE STEPS OF: HEATING SAID UNFINISHED STEEL ARTICLE TO A HOT-WORKING TEMPERATURE UNDER CONDITIONS WHICH, IN THAT PORTION OF THE UNFINISHED STEEL ARTICLE ADJACENT THE SURFACE THEREOF, (1) OXIDIZE AT LEAST SOME OF THE IRON THEREIN AND (2) CAUSE AN INCREASED CONCENTRATION OF AT LEAST ONE METALLIC ALLOYING ELEMENT OF THE STEEL OTHER THAN IRON; AND THEN DEFORMING SAID UNFINISHED STEEL ARTICLE WITH THE DEFORMATION BEING INITIATED WHILE SAID ARTICLE IS AT A HOT-WORKING TEMPERATURE; THE IMPROVEMENT COMPRISING THE STEP OF REMOVING THE ENTIRE SURFACE-ADJACENT PORTION OF SAID ARTICLE TO AT LEAST SUBSTANTIALLY THE DEPTH OF SAID INCREASED CONCENTRATION; SAID SURFACE-REMOVAL STEP BEING PERFORMED BETWEEN SAID HEATING AND DEFORMING STEPS; WHEREBY SURFACE CHECKING OF SAID ARTICLE, DURING SAID DEFORMING STEP, IS MINIMIZED. 